The detection of mechanical parameters such as the rate of rotation of a shaft has been accomplished in a variety of ways in the prior art, through the use of a wide range of magnetic, electromagnetic and optical systems. Such systems have often relied on magnetic field measurements such as the measurement of fields produced by the motion of a permanent magnet and by measurement of a magnetic field produced by the flow of electric current. In addition, numerous systems have been developed for optical measurement of motion, such as through the use of optically encoded shields or discs. Although such optical devices have been generally suitable for a wide variety of applications, there are conditions where such systems are not entirely satisfactory for they are usually extremely sensitive to alignment and vibration, and thus are often difficult to mount and adjust for accurate and reliable operation in adverse environmental conditions. Furthermore, many measuring systems are intrusive, in that they adversely affect the target being monitored or are adversely affected by the target environment, and many such systems are sensitivity to electromagnetic interference, thus limiting their usefulness.